Multi-antenna technology, which allows for simultaneous transmission/reception of wireless signals at a transceiver, is widely used in the 3rd Generation (3G) and Long Term Evolution (LTE) communication systems. To make a full use of the advantages of multi-antenna transceivers, it is required that transmission/reception chains of such a multi-antenna transceiver have the same signal response characteristic (including phase rotation characteristic and amplitude scaling characteristic). To satisfy this requirement, the so-called intra-transceiver antenna calibration process must be performed, whereby differences in phase rotation and amplitude scaling among the transmission/reception chains can be compensated for.
FIG. 1 illustrates an exemplary multi-antenna transceiver 10 according to the prior art, wherein an antenna array 110, a radio unit 120 and a Base Band Unit (BBU) 130 are provided. The antenna array 110 comprises four antenna elements. The radio unit 120 comprises four pairs of transmission (TX) chains 111, 112, 113 and 114 and reception (RX) chains 121, 122, 123 and 124 as well as a pair of a calibration transmission chain 11c and a calibration reception chain 12c.
Each of the transmission chains 111, 112, 113 and 114 refers to a respective signal transmission path from the BBU 130 to the antenna array 110 through the radio unit 120 for transmitting a respective transmission signal via a respective antenna element of the antenna array 110. Each of the reception chains 111, 112, 113 and 114 refers to a respective signal reception path from the antenna array 110 to the BBU 130 through the radio unit 120 for receiving a respective reception signal via a respective antenna element of the antenna array 110. Thus, the transmission and reception chains comprise a plurality of signal processing elements or stages (such as mixing stages, modulating or demodulating stages, filter stages, coding or decoding stages, amplifying stages, etc.) required for transmitting or receiving transmission signals and feeder cables connecting the radio unit 120 to the antenna array 110.
Depending on a transmission or reception calibration operation, respective switching elements S1, S2, S3 and S4 and a calibration switching element Sc are switched by a control function (not shown) to predetermined switching positions. In the case of reception calibration as shown in FIG. 1, the switching elements S1 to S4 are switched to the reception chains 121, 122, 123 and 124, so that the antenna elements are connected to respective reception chains 121, 122, 123 and 124 and the reception chains 121, 122, 123 and 124 can be calibrated. In the meanwhile, the calibration switching element Sc is switched to the calibration transmission chain 11c and connected to a combining or branching element 115 (e.g., passive power combiner/splitter), so that a calibration coupling path can be formed.
In the operation of reception calibration, a calibration signal is forwarded through the calibration transmission chain 11 and branched to the reception chains 121, 122, 123 and 124. By comparing the calibration signal and its distorted versions received through the reception chains 121, 122, 123 and 124, a calibration vector ({right arrow over (w)}r), whose elements are to be applied to the respective reception chains 121, 122, 123 and 124, may be determined for compensating the signal response characteristic differences among the reception chains 121, 122, 123 and 124. Because the signal response characteristic includes both the phase rotation characteristic and the amplitude scaling characteristic as described above, each element of the calibration vector ({right arrow over (w)}r) may accordingly contain an amplitude scaling calibration component x and a phase rotation calibration component y and be expressed as a complex exponential xejy.
In the case of transmission calibration, the switching elements S1, S2, S3 and S4 are switched to the transmission chains 111, 112, 113 and 114 and the calibration switching element Sc is switched to the calibration reception chain 12c. Orthogonal calibration signals are forwarded through the transmission chains 111, 112, 113 and 114, combined at the combining or branching element 115 and then forwarded to the calibration reception chain 12c. By comparing the orthogonal calibration signals and their distorted versions received through the calibration reception chain 12c, a calibration vector ({right arrow over (w)}t), whose elements are to be applied to the respective reception chains 121, 122, 123 and 124, may be determined for compensating the signal response characteristic differences among the transmission chains 111, 112, 113 and 114.
To further enhance system performance, the so-called Coordinated Multiple-Point Transmission (CoMP) technology has been introduced to LTE. As one of various CoMP schemes, the Joint Transmission (JT) scheme is characterized in that two or more transceivers are coordinated to jointly transmit data to a User Equipment (UE), as illustrated in FIG. 2. Such an arrangement not only increases desired signal power received at the UE but also reduces interference among the transceivers.
To guarantee coherent reception at the UE, it is not enough to just conduct the above-described intra-transceiver antenna calibration within each of the transceivers to compensate the signal response characteristic differences among the reception/transmission chains of the respective transceiver. Instead, inter-transceiver antenna calibration, which allows signal response characteristic differences among reference transmission/reception chains of different transceivers to be compensated for, is indispensable.
For the inter-transceiver antenna calibration, WO20110544144A1 and CN102149123A propose a solution called node-assistant inter-transceiver antenna calibration, wherein a wireless-enabled assistant node (such as a relay, a micro station or a UE) is introduced for inter-transceiver antenna calibration as illustrated in FIG. 3. In the case of inter-transceiver antenna reception calibration, transceivers involved in CoMP receive a calibration signal from the assistant node and calibrate their reference reception chains according to a comparison between the calibration signal and its distorted versions received at the transceivers. In the case of inter-transceiver antenna transmission calibration, the assistant node receives orthogonal calibration signals from the transceivers, determines calibration parameters for calibrating the reference transmission chains of the transceivers according to a comparison between the orthogonal calibration signals and their distorted versions received by the assistant node, and feeds the calibration parameters respectively back to the transceivers.
Thus, without the assistant node, it is impossible to implement the node-assistant inter-transceiver antenna calibration among the transceivers involved in CoMP. Moreover, as propagation paths between respective transceivers and the assistant node differ from each other, the distortions of the calibration signal by the propagation paths cannot be cancelled out for inter-transceiver antenna calibration, which is intended to compensate signal response characteristic differences among reference transmission/reception chains of different transceivers. As a result, the node-assistant inter-transceiver antenna calibration solution, which is based on the comparison between the calibration signal and its distorted versions received over the different propagation paths, inherently suffers from calibration inaccuracy.
In addition, to carry out the inter-transceiver antenna calibration, the node-assistant inter-transceiver antenna calibration solution requires one assistant node to be deployed for each CoMP set. For a typical radio network which may comprise hundreds or thousands of CoMP sets, efforts and costs incurred by deploying assistant nodes are considerable.
For the inter-transceiver antenna calibration, CN102843173A discloses another solution called transceiver-assistant inter-transceiver antenna calibration as illustrated in FIG. 4, wherein one of the transceivers involved in CoMP is selected to work as the assistant node in the node-assistant inter-transceiver antenna calibration solution. However, as the underlining principle of the transceiver-assistant inter-transceiver antenna calibration is the same as that of the node-assistant inter-transceiver antenna calibration, the distortions of the calibration signal by propagation paths between the selected transceiver and respective transceivers other than the selected one cannot be cancelled out, either. Consequently, the calibration inaccuracy still exists. In addition, the transceiver-assistant inter-transceiver antenna calibration cannot be implemented in a scenario where it is impossible to find a third transceiver that can work as an assistant node for only two transceivers involved in CoMP.